1. Field of the Invention
The present invention relates to a drying apparatus which dries sheets with high moisture content after printing, and a printing apparatus.
2. Description of the Related Art
In a printing apparatus, a sheet should be dried after becoming wet in an image formation process. The sheet may be dried naturally or in a forced manner for short time drying. Japanese Patent No. 2657571 discloses an apparatus in which a photosensitive sheet of a silver halide photosensitive material is subject to forced drying after becoming wet in a developing process. The disclosed apparatus includes an endless belt provided along a sheet travelling direction. The endless belt rotates in contact with a back surface of the sheet. Hot air is blown on the endless belt through an ejection port to increase a temperature of the belt. This means that the endless belt is heated by the hot air. The heated belt contacts the sheet which is being conveyed and accelerates drying.
A high-speed printing apparatus prints on several tens or hundreds of sheets per minute on a DIN A4-sized sheet basis. The sheets are conveyed at average speed of several millimeters to several centimeters per second. A drying apparatus should be capable of drying each sheet in several seconds and continuing the drying operation for a long time. However, it is difficult to use the apparatus disclosed in Japanese Patent No. 2657571 for high speed continuous printing for the following reasons.
In high-speed continuous printing, the temperature of the endless belt decreases gradually as drying time elapses as illustrated in a curve B of a temperature transition graph of FIG. 11. The decrease in the temperature of the endless belt is caused by the release of latent heat when the moisture evaporates from the sheet. The reason of the continuous decrease in temperature of the endless belt is as follows. During the continuous printing, the hot air blowing on the endless belt is blocked by the sheets and thus heating of the endless belt becomes insufficient. As a result, the temperature of the endless belt decreases continuously as the printing is continued for a long time, and when the temperature is below the lower limit permissive temperature for obtaining necessary drying performance (T-min), drying performance which is necessary cannot be exhibited.
If the temperature of the hot air which is blown on the endless belt is increased to achieve an increased initial temperature of the endless belt, the time until the curve B reaches the T-min may be prolonged. However, due to the upper limit permissive temperature of resistance to heat of the sheet (T-max), the temperature of the hot air cannot be increased excessively. The sheet used for printing is constituted by, for example, a receptive layer and a base film. The T-max of the sheet is, for example, 90 degrees C. and it is undesirable to heat the sheet to a temperature above the T-max.